The present invention relates generally to wastewater treatment. More particularly, the present invention relates to systems and methods for removing fluoride from wastewater using single fluoride sensing electrode.
Fluoride must be removed from wastewater generated at semiconductor fabrication and other industrial plants. This is an expensive process which is accomplished by adding calcium. The calcium is typically added either as lime or as calcium chloride solution which precipitates insoluble calcium fluoride. The wastewater which has been chemically treated to remove fluoride is typically either filtered or allowed to settle in specially designed settling tanks. One problem of prior art methods using lime or soluble calcium salts is the significant over-adding of the calcium to ensure that the fluoride is sufficiently precipitated. In the case of lime, excesses of 200-400% of the stoichiometric amount are commonly used. Despite these efforts however, fluoride removal is often less than adequate.
There has been no systematic study of high fluoride concentration (HFC) wastewater treatment. High fluoride concentration is defined as over 2000 ppm (2 g/l) fluoride and such high concentrations present additional problems for treatment. Accordingly, further developments in automatically and economically removing fluoride from wastewater, especially high fluoride concentration wastewater are needed.
It is an object of the present invention to provide a system and method for removing fluoride from wastewater continuously in compliance with discharging limits, while simultaneously allowing optimum use of calcium salts and providing good filtration. It is a further object of the present invention to provide a system and method that can be used for batch treatment of fluoride, continuous treatment of wastewater with random dumps of high fluoride concentration, and continuous treatment of wastewater with continuously varying high fluoride concentration.
In accordance with the present invention, there is provided a system for removing fluoride from wastewater. The system comprises a reaction tank for processing said wastewater by adding calcium salts, a filtration tank for removing the precipitated fluoride formed in the reaction tank, a single fluoride electrode disposed at a first input of the reaction tank for measuring a concentration of fluoride in the influent wastewater and providing an output signal, and a programmable controller for controlling addition of said calcium salts into said reaction tank. The programmable controller defines a setpoint of fluoride concentration in the reaction tank and automatically controls addition of calcium salts based on the setpoint and the output signal provided by the single fluoride electrode.
In another embodiment of the present invention, the system comprises a reaction tank for processing said wastewater by adding calcium salts, a filtration tank for removing the precipitated fluoride formed in the reaction tank, a first single fluoride electrode disposed at a first input of the reaction tank for measuring a concentration of fluoride in the influent wastewater and providing an output signal, a second single fluoride electrode disposed at the output of the filtration tank for monitoring a concentration of fluoride in the filtered wastewater and providing an output signal, and a programmable controller for controlling addition of said calcium salts into said reaction tank. The programmable controller defines a setpoint of fluoride concentration in the reaction tank and automatically controls addition of calcium salts based on the setpoint and the output signal provided by the first and second single fluoride electrode.
The reaction tank is preferably provided with a pH sensor for measuring pH of the influent wastewater fed into the reaction tank and providing an output signal. The pH sensor is connected to the controller which defines a setpoint of pH in the reaction tank and automatically controls addition of an acid or basic solution based on the pH setpoint and the output signal provided by the pH sensor.
In one embodiment, the reaction tank comprises at least a first, second, and third tank in series, and each tank is provided with a pH sensor for measuring pH in each tank. The first tank is added from stoichiometric to 125% stoichiometric of calcium salts, and said second and third tanks are added from 5 to 25% of the amount of calcium salts added to the first tank.
In another aspect of the present invention, there is provided a method of removing fluoride from wastewater. The method comprises defining a setpoint of concentration of fluoride using a programmable controller, measuring a concentration of fluoride in said wastewater using a first single fluoride electrode, automatically adding calcium salts to the wastewater in an amount based on said setpoint defined by the controller and the concentration of fluoride measured by the first single fluoride electrode, and monitoring a concentration of fluoride in the effluent wastewater using a second single fluoride electrode to ensure meeting discharging limits. Preferably, the calcium is added to the reaction tank in an amount of from stoichiometric to 125 percent of stoichiometric.
The method may further comprise defining a pH setpoint using the controller and measuring the pH of the wastewater using a pH sensor, and automatically adding an acid or basic solution to the wastewater in an amount based on the setpoint defined by the controller and the pH measured by the pH sensor. The pH setpoint is preferably defined at a value in a range from 6 to 9. A coagulant and/or flocculant may also be added to the wastewater to facilitate removal of precipitate fluoride.
The concentration of fluoride contained in the influent wastewater to be treated by the present method can be continuously varied, ranging from 50 to 10,000 ppm. The concentration of fluoride contained in the effluent water treated by present method can be as low as from 2 to 10 ppm.